1. Field of the Invention
The present invention relates to an MRI (magnetic resonance imaging) apparatus and a magnetic resonance imaging method which excites nuclear spin of an object magnetically with an RF (radio frequency) signal having the L armor frequency and reconstructs an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method, which can set imaging conditions for each of plural imaging regions and acquire data from the respective imaging regions according to the set imaging conditions.
2. Description of the Related Art
An MRI is an imaging method that excites an atomic nuclear spin of an object disposed in a static magnetic field by using an RF signal having a Larmor frequency and reconstructs the image on the basis of an NMR signal generated by the excitation.
In the field of magnetic resonance imaging, as a method of obtaining an image of a blood flow, MRA (magnetic resonance angiography) is known. MRI that does not use a contrast medium is referred to as a non-contrast-enhanced MRA. As non-contrast-enhanced MRA, an FBI (fresh blood imaging) method that performs an ECG (electro cardiogram) or peripheral pulse gating (PPG) synchronization to capture a pumping blood flow ejected from the heart is known, thereby satisfactorily representing a blood vessel (see, for example, Japanese Patent Application (Laid-Open) No. 2000-5144).
As non-contrast-enhanced MRA by the FBI method, an MRA image in which an artery and a vein are distinguished from each other is obtained by obtaining a difference between image data acquired by changing a delay time of ECG synchronization. Further, a flow-spoiled FBI method for suppressing an artery signal at systole by applying a spoiler pulse in the FBI method is known. According to the flow-spoiled FBI method, a difference between artery signals at diastole and systole of the cardiac cycle is imaged. In addition, an ECG-prep scan for determining an optimum delay time for ECG synchronization is known.
Further, in the FBI method, in order to extract blood flow having low flow velocity, a flow-dephasing method in which a gradient pulse (Gspoil) is applied in an RO (readout) direction, and a dephase pulse or a rephase pulse is applied as a gradient magnetic field pulse is known (see, for example, Japanese Patent Application (Laid-Open) No. 2002-200054, Japanese Patent Application (Laid-Open) No. 2003-135430 and U.S. Pat. No. 6,801,800). According to the flow-dephasing method, due to the dephase pulse or the rephase pulse, it is possible to increase relative signal difference between a signal value from blood flow having UM high velocity and a signal value from the blood flow having Hall low velocity. Therefore, it is possible to clearly distinguish an artery and a vein from each other on the basis of relative flow speed difference.
Furthermore, a technique for applying a t-SLIP (Time-SLIP: Time-Spatial Labeling Inversion Tagging Pulse) to selectively image or suppress only blood flowing into an imaging section is known (see, for example, Japanese Patent Application (Laid-Open) No. 2001-252263). In this t-SLIP method, a t-SLIP is applied with a constant delay time from an R wave of an ECG signal to label blood flowing into an imaging area. Consequently, signal intensity of blood that reaches an imaging section after a TI (inversion time) is enhanced.
Further, a technique to obtain dynamic state information of blood flow simply without a contrast medium and measuring an ECG-synchronization timing by the ECG-prep scan is known (see, for example, Japanese Patent Application (Laid-Open) No. 2004-329614). This technique uses an ECG-prep scan as an imaging scan. This means dynamic state information of blood flow can be obtained by subtraction processing to pieces of two-dimensional data acquired by plural acquisitions under an imaging scan, like an ECG-prep scan, while gradually changing a delay time from an R wave of an ECG signal.
Meanwhile, a receiver RF coil to receive an NMR signal may be used as an RF coil to transmit an RF signal. However, in many cases, a dedicated receiver RF coil according to an imaging region is used. For example, an array coil composed by aligning coil elements in a body axis direction is proposed as a coil for spine (see, for example, Japanese Patent Application (Laid-Open) No. H5-261081). In case of imaging an entire abdomen, multiple coil elements are arranged so as to encircle an object and NMR signals are received from the entire abdomen (see, for example, Japanese Patent Application (Laid-Open) No. 2003-334177).
However, since a coil element is necessary to be arranged with respect to every imaging region, there is a problem when the number of coil elements increases. In addition, a user needs to replace a set coil element to one suitable for the imaging region each time an object or an imaging region changes. For this reason, a user needs to prepare many dedicated coil elements suitable for imaging regions, and replacement of a coil element is a very onerous task for a user such as a doctor and an engineer in the field.
Therefore, a technique has been designed to provide a switching circuit and/or a synthetic circuit (matrix) regarding multiple coil elements lined up in an X-axis direction perpendicular to a body axis of an object to enable modal selection of combinations of coil elements used for receiving (see, for example, Japanese Patent Application (Laid-Open) No. 2003-334177).
The conventional blood flow imaging is performed per section by moving a receiver RF coil specialized for an imaging region in contrast to imaging an organ and an internal organ. That is, a user moves a receiver RF coil to a position suitable for imaging blood flow in a section to be a next target after imaging blood flow with regarding to a certain section. Then, after a position of the RF coil is determined, blood flow in a corresponding section is imaged.
Imaging blood flow can be performed over multiple imaging regions where dedicated receiver RF coils mutually differ. In this case, a receiver RF coil needs to be replaced each time an imaging region changes. Especially in imaging blood flow throughout an entire body, an RF coil needs to be replaced frequently.
Furthermore, when an imaging sequence suitable for imaging blood flow varies every imaging region, a user needs to reset an imaging sequence each time an imaging region changes.
Thus, in imaging widespread blood flow, in addition to an onerous task such as selection and placement of a receiver RF coil, a user may encounter a troublesome operation such as resetting an imaging sequence. These are common problems in acquiring a widespread image as well as imaging blood flow.